U.S. Pat. No. 3,127,375 to Keck relates to the use of columbium (now called niobium) compounds as catalysts for ester interchange reactions between glycols and esters of polycarboxylic acids or mixtures of esters of such acids. The patent states that a polyester is formed which has good color, that is freedom from discoloration, and the examples state the melt color is yellow.
U.S. Pat. No. 3,758,438 to Freitag relates to the utilization of niobium as in the form of niobium pentoxide to impart a golden hue to the polyester.
Neither of these patents, however, suggest the production of a green colored polyester, or of the utilization of the niobium catalysts with titanium pigments or catalysts together with phthalocyanine blue to obtain a green color.